Fix bad index reference. Add new ones.

Author: rouilj

doc/reference.txt

@@ -664,6 +664,8 @@ A tracker schema defines what data is stored in the tracker's database.
 Schemas are defined using Python code in the ``schema.py`` module of your
 tracker.
 
+.. index:: schema; allowed changes
+
 What you can/can't do to the schema
 -----------------------------------
 
@@ -753,7 +755,7 @@ specifying (default) labelling and ordering of classes.)::
         priority=Link("priority"))
     issue.setkey('title')
 
-.. index:: schema; allowed changes
+.. index:: schema; classes and properties
 
 Classes and Properties - creating a new information store
 ---------------------------------------------------------
@@ -799,6 +801,10 @@ users) for its classes. Most REST documentation suggests using plural
 forms. However, to make your API consistent, use singular forms for
 classes that you add.
 
+.. index::
+   schema; classes
+   schema; items
+
 Class and Items
 ~~~~~~~~~~~~~~~